Internal body organs may be imaged and otherwise characterized by apparatus which directs pulses of ultrasound energy into the body and subsequently detects echoes which originate when the energy is reflected from tissue interfaces or other discontinuities within the body. In typical apparatus the ultrasound energy is directed into the body in a relatively narrow beam. Electric signals which describe the position and direction of the beam with respect to the body, as well as the relative arrival time and amplitude of the echos, are utilized to generate a visual display and/or mapping of the internal body structures. In many applications the direction of the ultrasound beam is manually controlled by a technician (generally by physical motion of a probe head) to build up a display pattern. While these methods are adequate for imaging stationary body structures, the time required for physical motion of a probe is generally much too long to image rapidly moving body structures (for example the valves in a beating heart) in real time. Ultrasound systems for generating real time displays of rapidly moving body organs generally utilize electromechanical or electronic means to change the position and direction of one or more beams of ultrasound energy with respect to the body.
Motion of a beam of ultrasound energy with respect to the body may be provided by sequentially activating transducer elements in a flat linear array to effectively scan an area of the body with a sequence of substantially parallel ultrasound beams. A device of this type is described in U.S. Pat. No. 3,013,170. A beam of ultrasound energy may, alternately, be scanned around a single origin point to produce a so-called "sector-scan." Sector-scan geometries are particularly useful since ultrasound energy may be directed between the ribs to scan the interior of the chest cavity. Sector-scanning has been achieved in the prior art by rapidly rotating one or more transducers about an axis, by steering energy from a fixed transducer with a rotating ultrasound reflector, or by sequencing individual transducer elements in a linear curved array. British Pat. No. 1,546,445 describes a curved transducer array with individual transducers which are individually activated to produce a sector-scan.
The transverse spatial resolution which may be obtained from a sequence array of ultrasound transducers is related to dimensions of the individual transducer elements in the array. Small transducer elements are desirable for obtaining fine resolution. The amount of ultrasound energy produced by an individual transducer element is, however, limited by its size. The signal-to-noise ratio of the returned ultrasound echoes necessarily depends on the amount of ultrasound energy introduced into the body. Thus, the signal to noise ratio suffers if small transducer elements are individually activated to achieve a scanning action. Diffraction effects will, furthermore, cause spreading of an ultrasound beam which originates from a single, small ultrasound transducer element.
This problem has been solved in the prior art by simultaneously activating a group of adjacent transducers within a flat linear array. Means were provided for incrementally shifting the active group along the array to provide fine spatial resolution and high signal-to-noise ratios. While this technique is appropriate for use with flat transducer arrays, which produce a parallel beam scanning geometry, the simultaneous activation of a group of adjacent transducers in a curved array inherently generates a focussed ultrasound beam. Sequenced group arrays have not, therefore, found application for the generation of high resolution sector-scans.